Page 187 - Carbonate Facies in Geologic History

174 Pennsylvanian-Lower Permian Shelf Margin Facies

West Texas, at Middle Pennsylvanian shelf margins, it is known to form porous grainstones
and packstones on tops and seaward sides of buildups. Surprisingly, it has not been reported
from extensively exposed and studied Middle Pennsylvania strata from the Paradox basin.

Palaeoaplysina

The form constitutes a genus, also described in Russia, which has recently been identified in
the Canadian Arctic (Davies, 1971; Davies and Nassichuk, 1973) forming important bio-
herms of early Permian age. This organism is tentatively assigned to the hydrozoans and bears
some similarity to stromatoporoids. It is described as forming thin plates or tabular pieces a
few cm long, with internal branching tubules, a cellular skeleton and external protuberances.
The plates occur in micritic matrix and also in debris piles as grainstones with platy algae and
Tubiphytes boundstone. Mounds exist up to a few tens of meters thick and a hundred meters
across. They may be stacked or offiap each other. The Palaeoaplysina mounds are known in
the western Urals (as oil reservoirs), Idaho, the Yukon, and on both sides of the Sverdrup
basin in Ellesmere Island outcrops. Like Tubiphytes the organism is known in Late Pennsyl-
vanian strata but is more common in Early Permian beds. It is a member of the shallow shelf
community, best developed in mounds of basin margin carbonates and clastics. Associated
biota (foraminifera and algae) is that normally found in Late Paleozoic carbonates. Abundant
dasycladaceans and oolites in such buildups indicate shallow water. Palaeoaplysina mounds
are also interbedded with clastics in more basinal situations. Its distribution in the northern
areas of both hemispheres and its apparent absence in the well-known Late Paleozoic of the
southwestern U.S.A. indicates. that it is a member of the boreal fauna of Permo-Pennsyl-
vanian age.

Encrusting Organisms

A veneer of encrusting organisms exists on the tops of some micritic bioherms in the south-
western U.S.A. Some of these were mentioned by Parks (1962), but the assemblage has never
been described. Such fossils are commonly silicified and poorly preserved. Arcuate stromato-
litic forms, stromatoporoids?, and sponges are known. The coarse fabric of the sponge
Stereodictyon is recognizable at the top of the mounds in both the Sacramento and Big
Hatchet Mountains of New Mexico.

Examples of Permo-Pennsylvanian Carbonate Buildups

The above described biotas and eleven basic microfacies described in Chapter VII
comprise several types of carbonate buildups which exist in various tectonic
settings throughout the southwestern U.S.A. Many have been carefully studied
from outcrops and, as well, in the subsurface where the buildups provide extensive
petroleum reservoirs. Several of these are described and figured below.

Middle Pennsylvanian Beds of the Carbonate Shelf Facies, Paradox Basin

Narrow but well-defined biohermal trends were first described and mapped in the
Paradox basin by Wengerd (1951,1955,1963) from outcrops in the Goosenecks of
the San Juan River where it crosses the Monument Upwarp in Utah. Tectonically